1. Field of the Invention
The present invention relates to an image reading apparatus for optically reading an image carried on a subject that is placed on a subject holder, and more particularly to the structure of a sensor displacing unit in such an image reading apparatus, for displacing an image reader which essentially reads an image, in the main scanning direction of a line sensor whose signal transfer unit may comprise a CCD or a signal-charge amplifying array of MOS transistors. The image carried on the subject refers to an image that has been formed on the subject by a physical action or a chemical action or a combined physical and chemical action.
2. Description of the Related Art
There have widely been used image reading apparatus for reading an image on a subject with a CCD (Charge-Coupled Device) to generate an image signal. The CCD comprises an array of photoelectric transducers for converting light into electric signals, the photoelectric transducers serving to divide the image into pixels and read the image as pixel signals.
The resolution of the image reading apparatus using the CCD is limited by the number of photoelectric transducers of the CCD. Various techniques have been proposed in the art to increase the resolution while the number of photoelectric transducers used remains unchanged.
For example, Japanese laid-open patent publication No. 3-236687 discloses an area sensor comprising solid-stage imaging elements. The solid-stage imaging elements are displaced in four directions, i.e., upward, downward, leftward, and rightward directions, and pixel signals generated by the solid-stage imaging elements when they read an image in the displaced positions are arranged into a predetermined pattern for thereby increasing the apparent number of pixels provided by the solid-stage imaging elements to achieve an increased resolution.
According to the disclosure of Japanese laidopen patent publication No. 7-283915, a line sensor whose signal transfer unit comprises a CCD is displaced in a main scanning direction along which pixels are arrayed, i.e., signal charges are transferred, for thereby increasing the apparent number of pixels with respect to the main scanning direction. The apparent number of pixels in an auxiliary scanning direction, which extends perpendicularly to the main scanning direction, may be set to a value corresponding to the apparent number of pixels in the main scanning direction by adjusting the speed at which the subject with the image carried thereon is fed in the auxiliary scanning direction. Since the line sensor has a one-dimensional array of photoelectric transducers, it is easier to increase the number of photoelectric transducers than with an area sensor which has a two-dimensional array of photoelectric transducers. Therefore, the line sensor is suitable for use in image reading apparatus which require a high resolution.
When an image is read by a linear sensor such as a CCD linear image sensor comprising photoelectric transducer pixels that are coupled in a main scanning direction, the resolution at which the image is read by the linear sensor basically depends upon the size of each of the photoelectric transducer pixels. The size of each of the photoelectric transducer pixels is defined as the distance between adjacent ones of the photoelectric transducer pixels, e.g., the distance between the centers thereof (referred to as a "pixel pitch").
In order to increase the reading resolution of a linear sensor, it has been proposed to displace the linear sensor back and forth a minute distance in a main scanning direction with a piezoelectric device. Such a process is known as a pixel shifting process. A pixel shifting process as applied to a linear image sensor is disclosed in Japanese laid-open patent publication No. 7-283915.
According the disclosed process, when an image is read by the linear image sensor line by line in the main scanning direction, the linear image sensor is displaced in the main scanning direction by a distance M=p.multidot.q.multidot.2/(2.multidot.m+1) where p represents the distance (pixel pitch) between the centers of adjacent ones of the pixels, m represents an integer of 1 or more, and q an integer that is changed successively for the respective lines in the range of from 0 to 2 m. After the resolution is increased up to (2.multidot.m+1) times, a moving average is calculated of successive (2.multidot.m+1) pixel signals thus producing pixel signals of the increased resolution.
For example, if m=1, then the displaced distance M is M=0 when the integer q is q=0, the displaced distance M is M=p.multidot.2/3, i.e., a 2/3 pixel, when the integer q is q=1, and the displaced distance M is M=p.multidot.4/3, i.e., a 4/3 pixel, when the integer q is q=2. Therefore, the linear image sensor is shifted a 2/3 pixel each time, increasing the resolution three times. The moving average of three pixel signals is calculated in order to smooth level differences between pixel signals from odd- and even-numbered photoelectric transducers if the linear image sensor comprises odd- and even-numbered pixel transfer units.
With the pixel shifting process using the piezoelectric device, the line sensor and the piezoelectric device may be housed in a case and sealed together by a resin body, or may be combined together by a molded resin body. In either case, the line sensor and the piezoelectric device are combined as a single unit.
However, when either one of the line sensor and the piezoelectric device is damaged, the line sensor and the piezoelectric device that are unitized must be replaced with a new unit even though the other of the line sensor and the piezoelectric device may still be usable. Such a replacement practice possibly imposes a high running cost on the user, resulting in a bottleneck in efforts to promote widespread use of the image reading apparatus.
In unitizing the line sensor and the piezoelectric device, the positional relationship between the line sensor and the piezoelectric device, particularly, the distance by which the piezoelectric device projects toward the line sensor, tends to differ from apparatus to apparatus. As a result, the distance that the line sensor is displaced also varies from apparatus to apparatus, possibly causing quality deteriorations of reproduced images.
When the unit of the line sensor and the piezoelectric device is installed in an image reading apparatus, optical positional adjustments are usually effected on the unit. Since the distance by which the piezoelectric device projects toward the line sensor tends to differ from apparatus to apparatus, the operator has to make optical positional adjustments again while viewing a reproduced image. This process makes it complex and needs an increased number of steps to install the unit in the image reading apparatus.
Another problem of the pixel shifting process using the piezoelectric device is that when a drive potential is applied to the piezoelectric device to vibrate it to displace the CCD, the displacement of the CCD becomes stable only a certain period of time after the piezoelectric device has started to be energized.
One solution would be to ignore, for the purpose of signal processing, image signals that are outputted from the line sensor during a period of time (settling time) before the displacement of the CCD becomes stable. According to this solution, however, the period of time in which no image is actually read is relatively long in the total scanning time, possibly impairing the reproducibility of the image.
In the pixel shifting process using the piezoelectric device, the linear image sensor is displaced minute distances by switching the drive voltage applied to the piezoelectric device. To produce a high-quality image signal, it is necessary to read the image after the vibration of the piezoelectric device and hence the linear image sensor is settled to a certain extent.
Heretofore, it has been customary to suppress the vibration of the piezoelectric device and hence the linear image sensor with a mechanical component such as a spring. However, since a certain amount of vibration still remains unsuppressed, the storage of charges in the photoelectric transducer pixels is made effective only after the vibration is settled after the linear image sensor is energized.